Cytochrome P-450 is a major component of a mixed-function oxidase enzyme system which catalyzes the metabolism of many compounds, including polycyclic hydrocarbons known to produce cancer in humans. Several forms of cytochrome P-450 differing in activity and substrate specificity are known to exist in mammalian liver. The ability of the individual to activate or de-activate chemical carcinogens and thus his susceptibility to cancer induction by a given agent may depend on the absolute amounts or relative proportions of particular cytochrome P-450 species present. Hence, it would be useful to be able to recognize and quantitate the cytochrome P-450 forms. In this study, methods for the identification of these proteins have been developed using rat and rabbit liver microsomes and currently are being used to analyze cytochromes P-450 of human monocytes. The methods employed effectively combine isoelectric focusing, column chromatography, enzymatic assay, SDS-polyacrylamide gel electrophoresis and spectrophotometry and should be useful for examining cytochrome P-450 content of low activity as well as high activity tissues.